This invention relates to equipment for processing substrates, and in particular to a method and apparatus for processing two or more substrates at the same time in a single processing chamber without a corresponding increase in the processing chamber footprint.
Numerous techniques and apparatus are well-known for use in the processing of semiconductor wafers to make integrated circuits. State of the art fabrication facilities (known as "fabs") for carrying out such processes are typically large buildings within which "clean rooms" of thousands of square feet of floor area are provided. The clean rooms contain the equipment within which the various semiconductor fabrication processes are carried out, for example, chemical vapor deposition equipment for deposition of conductive or insulative materials on the wafers, ion implantation equipment for introduction of dopants into the wafers, furnaces for heating the wafers, plasma etchers for removing material from the wafers, etc.
Compared with even their recent predecessors, clean rooms today are extraordinarily clean, often having particle densities of less than class 1. Such low particle densities require expensive equipment to purify the air in the clean room, as well as unusual care in all other respects. The result of these measures is that floor space in such clean rooms is expensive. The per-square-foot construction cost, as well as maintenance cost, is high.
A trend in the manufacture of integrated circuits is the use of single wafer processing equipment. In single wafer equipment, processing is carried out on the wafers one wafer at a time. That is, one wafer is introduced from a cassette holding many wafers into the processing chamber. The necessary process on the wafer is carried out in the chamber, then the wafer is removed from the chamber and the next wafer introduced. Typically, such single wafer processing chambers are clustered around a central robot that can load the chambers with individual wafers. The use of single wafer processing provides higher yields by making the process more controllable across the entire wafer, typically 8 inches (20 cm) in diameter at present, and 12 inches (30 cm) in the near future. The higher yields produced by single wafer systems have resulted in their use in many of the advanced fabrication facilities used today in the semiconductor industry.
In such single wafer processing chambers the wafer is maintained on a pedestal or susceptor during the desired operation. An electrostatic chuck is sometimes used to hold the wafer in position. Using electrostatic forces to hold the wafer (or substrate) in place eliminates the need for a clamp ring, which is used in some other systems, to secure the substrate to the chuck.
Two types of electrostatic chucks used in processing chambers are unipolar and bipolar electrostatic chucks. Unipolar electrostatic chucks attract semiconductor wafers, or other substrates, to their dielectric-covered surfaces by the electrostatic attraction of induced charges. An induced charge is created by applying a voltage to the chuck and creating a charged plasma above the wafer as the current ground path. An example of a unipolar electrostatic chuck is shown in U.S. Pat. No. 5,459,632, entitled "Releasing a Workpiece From an Electrostatic Chuck". Bipolar electrostatic chucks do not require plasma to work. They are, however, more complicated to design and manufacture than unipolar chucks. An example of a bipolar electrostatic chuck is shown in U.S. Pat. No. 5,001,594, entitled "Electrostatic Handling Device".